1. Field of the Invention
This invention relates to a flake graphite cast iron suitable for use in engine parts of an internal combustion and the like and the production method thereof, which makes it possible to produce at a low price a cast iron being highly strong and excellent in workability such as cutting performance without using a misch metal and the like in particular.
2. Description of the Related Art
Conventionally, various studies have been done on engine parts of an internal combustion, wherein strength, stability of workability and cost reduction are required. For example, engine parts such as a cylinder liner or a piston ring are strongly required to have excellent wear resistance and scuffing resistance, since the piston ring needs to slide in the inner peripheral surface to maintain airtightness, so a special alloy cast iron having the structure that a graphite and a carbide are dispersed has been conventionally used. Although a cast iron with the addition of an alloy element such as molybdenum to enhance strength is cited as a special alloy cast iron, there is a problem that such a cast iron is poor in workability, shortens a cutting tool life and increases the working cost. Another problem is that the large addition of an expensive alloy element incurs an increase in the product cost.
In the production of the cast iron, from a viewpoint of effective use of resources, steel sheet scrap (steel scrap) is commonly used as a part of raw materials. In recent years, a high-tensile steel tends to be largely used as automobile materials for the purpose of collision safety improvement and vehicle weight reduction.
As an alloy element to add to the high-tensile steel, matrix structure strengthening elements such as Mn, Cr and Mo are cited. Among others, manganese is a low price and used most so that a huge amount of high-tensile scrap (steel scrap) containing a large amount of Mn is expected to be generated along with increase in the usage ratio of the high-tensile steel in the future.
Now, steel sheet scrap (steel scrap) is largely used as raw materials for a cast metal and a large amount of burrs and the like are generated as the steel scrap during pressing. Therefore, the high-tensile scrap mentioned above is expected to be used as an iron source.
Mn as one of the alloy elements hugely contained in the high-tensile scrap serves as a matrix structure strengthening element of stimulating pearlitic structure of a matrix and densifying spacing on cementite in the pearlite to strengthen it in a cast iron. However, Mn has a function of stabilizing a carbide to prevent crystallization of a graphite.
Therefore, at present, in order to produce a cast iron by using high-tensile scrap, steps such as dilution of Mn and removal of Mn are required. Since these steps involves increase in cost, it would be industrially very effective if a cast iron can be obtained directly without removing Mn.
Approximately 5 million tons of cast iron is produced a year in Japan. A cast iron of a graphite crystallized in flake state is called a flake graphite cast iron, approximately 3 million tons of which is produced a year, while a cast iron of a graphite crystallized in spheroidal state is called a spheroidal graphite cast iron, approximately 2 million tons of which is produced a year.
Moreover, the flake graphite cast iron generally has lower tensile strength than the spheroidal graphite cast iron. The main reason why the spheroidal graphite cast iron has higher tensile strength is that a graphite spheroidizing agent containing Mg, Ca, Ce and the like is added to a molten metal to spheroidize the graphite.
However, it has also become clear that sulfur (S) existing in the molten metal reacts with these elements to form a sulfide to thereby deteriorate graphite spheroidization.
Therefore, it is required to subject a molten metal containing a large amount of S to a treatment such as desulfurization treatment for lowering the amount of S or addition of a large amount of the graphite spheroidizing agent in advance. Since even a small amount of elements Sb, Sn, Pb, Ti and the like inhibits spheroidizing of the graphite, it is also required to remove these elements from raw materials such as scrap to make sure that these elements are not mixed therein.
Thus, while the spheroidal graphite cast iron can be produced to obtain a cast iron having high tensile strength, when the spheroidal graphite cast iron is produced from scrap, meticulous care and treatment are required for commingling of various elements and quantitative management of S, which generates a slag, is particularly important.
On the other hand, when the flake graphite cast iron is produced by using scrap, although much care is not required for the commingling of various elements as compared to the spheroidal graphite cast iron, it is generally difficult to obtain a flake graphite cast iron having tensile strength in the same level with the spheroidal graphite cast iron.
Moreover, S, which bonds to an iron (Fe) to become FeS to stimulate chilling when an amount of Mn is small in a cast iron, is generally recognized as a strong anti-graphitization element.
However, when Ms and S coexist in the molten metal of the cast iron, a stable sulfide (MnS) is formed and the ill effects of each are neutralized by each other. Furthermore, a possibility that MnS stimulates the nucleation of a graphite eutectic crystal is suggested.
While a study has been done on independent Mn or S, or a mutual relationship between the both elements, few studies assuming a high Mn composition are found other than the methods disclosed in Japanese Patent Application Laid-open Publication Nos. 2003-171729 and H10-158777 for example, which the subject inventors propose.
In both the methods disclosed in Japanese Patent Application Laid-open Publication Nos. 2003-171729 and H10-158777, wherein it is essential to add a rare-earth element or a misch metal twice as much as the amount of S in the molten metal, since the addition of the rare-earth metal or the misch metal deteriorates fluidity (melt fluidity of cast metal), there is a problem that work operation takes time and effort. Particularly a demand for thinning a cylinder liner, a camshaft and the like becomes severe to minimize the process and an excellent fluidity is required to satisfy such a demand.
Additionally, although the method disclosed in Japanese Patent Application Laid-open Publication No. 2003-171729 adopts the configuration that S is further added to the molten metal, since S causes an occurrence of the slag, which becomes an obstacle to reuse, the addition of S is not preferable.